Firstly, Invalidity Pension is based on PRSI contributions and is not means-tested, so rental income would not affect the pensions.
Property not used or enjoyed by the owner(s) is assessable for both medical card & rent supplement - if rented, the rental income is assessed, if not rented, the capital value of the property is assessed.
Is there space at the son's house for an extension/granny flat?
Could the couple do a deal with the local authority to sell the house to them, in return for some LA accommodation if it were available close to the required area? ( I know this has happened in Dublin in the past, person sold house to LA in return for a senior citizen's flat in the locality).
Could the parents move closer to the couple perhaps?